Minimum voltage detector circuit

ABSTRACT

A minimum voltage detector circuit is disclosed. The circuit includes a plurality of LED strings each having a plurality of series-coupled LEDs. The minimum voltage detector circuit is configured to detect a minimum voltage from among the plurality of LED strings, and also to perform open/short detection among the plurality of LED strings. The minimum voltage detector circuit includes a plurality of voltage comparators and correspondingly coupled replica circuits. Each of the voltage comparators includes an amplifier having a first input coupled to a cathode of a last LED of one of the plurality of LED strings, an output, and a second input coupled to the output. Each voltage comparator further includes a replica circuit coupled to the amplifier. The replica circuit is configured to maintain an output transistor of the amplifier in an active state when the amplifier is in an unbalanced state.

BACKGROUND Technical Field

This disclosure is directed to electronic circuits, and moreparticularly, to circuits for detecting a minimum voltage output from anumber of different circuits.

Description of the Related Art

Minimum voltage detection is a function that is often performed inmulti-string light-emitting diode (LED) applications. Each LED stringmay include a number of series coupled LEDs. The LED strings may be usedto, e.g., implement a backlight for a display used in various devices,such as smartphones and tablet computers. Detection of the minimumvoltage may allow for greater efficiency while performingstring-to-string current matching. Furthermore, detection of the minimumvoltage may allow a DC-DC converter (e.g., a boost converter) toregulate a supply voltage provided to each of the LED strings.

A minimum voltage detector circuit is disclosed. In one embodiment, acircuit includes a plurality of light-emitting diode (LED) strings eachhaving a plurality of series-coupled LEDs. The circuit further includesa minimum voltage detector circuit having a plurality of voltagecomparators. The minimum voltage detector circuit is configured todetect a minimum voltage from among the plurality of LED strings. Theminimum voltage detector circuit includes a plurality of voltagecomparators. Each of the voltage comparators includes an amplifierhaving a first input coupled to a cathode of a last LED of one of theplurality of LED strings, an output, and a second input coupled to theoutput. Each voltage comparator further includes a replica circuitcoupled to the amplifier. The replica circuit is configured to maintainan output transistor of the amplifier in an active state when theamplifier is in an unbalanced state.

In one embodiment, a DC-DC converter (e.g., a boost converter) iscoupled to provide a supply voltage to an anode of a first diode in eachof the LED strings. The DC-DC converter further includes controlcircuitry coupled to receive the minimum voltage detected from theminimum voltage detector. The DC-DC converter may control the supplyvoltage based at least in part on the minimum voltage.

In one embodiment, a fault detection circuit may also be implemented.The fault detection circuit may be coupled to the minimum voltagedetector circuit. The fault detection circuit may detect the presence ofa short circuit or an open circuit in a faulty LED string.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description makes reference to the accompanyingdrawings, which are now briefly described.

FIG. 1 is a block diagram of one embodiment of a voltage comparatorcircuit and its arrangement within in a minimum voltage detectioncircuit.

FIG. 2 is a schematic diagram of one embodiment of a voltage comparatorcircuit.

FIG. 3 is a diagram illustrating one embodiment of a minimum voltagedetection circuit.

FIG. 4 is a schematic diagram illustrating one embodiment of a faultdetection circuit.

FIG. 5 diagram illustrating one embodiment of a backlight system havinga number of LED strings and a DC-DC converter.

FIG. 6 is a flow diagram illustrating one embodiment of a method foroperating a voltage comparator circuit.

FIG. 7 is a block diagram of one embodiment of an example system.

Although the embodiments disclosed herein are susceptible to variousmodifications and alternative forms, specific embodiments are shown byway of example in the drawings and are described herein in detail. Itshould be understood, however, that drawings and detailed descriptionthereto are not intended to limit the scope of the claims to theparticular forms disclosed. On the contrary, this application isintended to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the disclosure of the presentapplication as defined by the appended claims.

This disclosure includes references to “one embodiment,” “a particularembodiment,” “some embodiments,” “various embodiments,” or “anembodiment.” The appearances of the phrases “in one embodiment,” “in aparticular embodiment,” “in some embodiments,” “in various embodiments,”or “in an embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Within this disclosure, different entities (which may variously bereferred to as “units,” “circuits,” other components, etc.) may bedescribed or claimed as “configured” to perform one or more tasks oroperations. This formulation—[entity] configured to [perform one or moretasks]—is used herein to refer to structure (i.e., something physical,such as an electronic circuit). More specifically, this formulation isused to indicate that this structure is arranged to perform the one ormore tasks during operation. A structure can be said to be “configuredto” perform some task even if the structure is not currently beingoperated. A “credit distribution circuit configured to distributecredits to a plurality of processor cores” is intended to cover, forexample, an integrated circuit that has circuitry that performs thisfunction during operation, even if the integrated circuit in question isnot currently being used (e.g., a power supply is not connected to it).Thus, an entity described or recited as “configured to” perform sometask refers to something physical, such as a device, circuit, memorystoring program instructions executable to implement the task, etc. Thisphrase is not used herein to refer to something intangible.

The term “configured to” is not intended to mean “configurable to.” Anunprogrammed FPGA, for example, would not be considered to be“configured to” perform some specific function, although it may be“configurable to” perform that function after programming.

Reciting in the appended claims that a structure is “configured to”perform one or more tasks is expressly intended not to invoke 35 U.S.C.§ 112(f) for that claim element. Accordingly, none of the claims in thisapplication as filed are intended to be interpreted as havingmeans-plus-function elements. Should Applicant wish to invoke Section112(f) during prosecution, it will recite claim elements using the“means for” [performing a function] construct.

As used herein, the term “based on” is used to describe one or morefactors that affect a determination. This term does not foreclose thepossibility that additional factors may affect the determination. Thatis, a determination may be solely based on specified factors or based onthe specified factors as well as other, unspecified factors. Considerthe phrase “determine A based on B.” This phrase specifies that B is afactor that is used to determine A or that affects the determination ofA. This phrase does not foreclose that the determination of A may alsobe based on some other factor, such as C. This phrase is also intendedto cover an embodiment in which A is determined based solely on B. Asused herein, the phrase “based on” is synonymous with the phrase “basedat least in part on.”

As used herein, the phrase “in response to” describes one or morefactors that trigger an effect. This phrase does not foreclose thepossibility that additional factors may affect or otherwise trigger theeffect. That is, an effect may be solely in response to those factors,or may be in response to the specified factors as well as other,unspecified factors. Consider the phrase “perform A in response to B.”This phrase specifies that B is a factor that triggers the performanceof A. This phrase does not foreclose that performing A may also be inresponse to some other factor, such as C. This phrase is also intendedto cover an embodiment in which A is performed solely in response to B.

As used herein, the terms “first,” “second,” etc. are used as labels fornouns that they precede, and do not imply any type of ordering (e.g.,spatial, temporal, logical, etc.), unless stated otherwise. For example,in a register file having eight registers, the terms “first register”and “second register” can be used to refer to any two of the eightregisters, and not, for example, just logical registers 0 and 1.

When used in the claims, the term “or” is used as an inclusive or andnot as an exclusive or. For example, the phrase “at least one of x, y,or z” means any one of x, y, and z, as well as any combination thereof.

In the following description, numerous specific details are set forth toprovide a thorough understanding of the disclosed embodiments. Onehaving ordinary skill in the art, however, should recognize that aspectsof disclosed embodiments might be practiced without these specificdetails. In some instances, well-known circuits, structures, signals,computer program instruction, and techniques have not been shown indetail to avoid obscuring the disclosed embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

A minimum voltage detector circuit is disclosed. Minimum voltagedetection is an important function in multi-string LED applications,such as in a display backlight. Each LED string in a multi-stringapplication may have its own LED driver circuit coupled to a cathode ofa last LED in the string, from which a voltage may be detected. Due tomismatches in the diodes, the voltage from one string to the next may bedifferent. An anode of a first diode in each LED string may be coupledto receive a supply voltage from a DC-DC converter (e.g., a boostconverter). Accordingly, the voltage drops across the LED strings arenot all the same when mismatches are present. The output voltage of theDC-DC converter may be regulated at least in part based on the voltagesdropped by each of the LED strings. Accordingly, detection of theminimum voltage among the ends of various LED strings may be animportant factor in determining the desired output voltage provided bythe DC-DC converter.

In addition to detecting a minimum voltage among a number of LEDstrings, string diagnoses is also a desirable function. From time totime, faults may occur in one or more LEDs in an LED string. A fault mayoccur as a short circuit or an open circuit. In the case of an opencircuit, the LED driver associated with the LED string in which thefault occurred may be shut down. In the case of a short circuit, theentirety of the backlight may be powered down.

The present disclosure contemplates a minimum voltage detection circuitand a voltage comparator circuit for implementing the same. In previousminimum voltage detection circuits, the number of LED strings for whichthe minimum voltage detected may be limited. Using the voltagecomparator circuit disclosed herein, a minimum voltage detector may beimplemented for a significantly greater number of LED strings. Thecircuitry implemented here may also include fault detection circuitryusable to detect faults within the LED strings.

Turning now to FIG. 1, a block diagram of one embodiment of a voltagecomparator circuit and its arrangement within in a minimum voltagedetection circuit is shown. In the embodiment shown, each of a number ofLED strings 120 include a first diode having its anode coupled toreceive a supply voltage, VddB. This supply voltage may be providedfrom, e.g., a boost converter or more generally, a DC-DC converter, asdiscussed in further detail below. A cathode of a last diode in each ofthe LED strings is coupled to a corresponding instance of a voltagecomparator 105 of minimum voltage detector 100. In particular, thecathode of the last diode in each LED string 120 is coupled to a firstinput, Vin1, of a corresponding one of the voltage comparators 105. Thenumber of voltage comparators 105 implemented in minimum voltagedetector 100 may vary from one embodiment to the next.

Each of the voltage comparators 105 includes an output, Vout, that iscoupled to the output node of each of the other ones of the voltagecomparators 205. The minimum voltage may be detected and determined onthis node, which is commonly labeled as Vmin. The output node of each ofthe voltage comparators 105 is also coupled to a second input, Vin2, ofthe same comparator.

The internal arrangement of each voltage comparator 105 may correspondto that shown in the instance shown in the lower left hand corner. Inparticular, each voltage comparator 105 may include an amplifier 112 anda replica circuit 114. The amplifier 105 in each instance may includethe first and second inputs, Vin1 and Vin2, along with the output, Vout.The amplifier in each instance may include an output transistor, Mout,with the amplifier output, Vout, taken from the drain of this device.The replica circuit 114 may ensure that the output transistor, Mout,remains on when the corresponding voltage detected on its Vin1 input isnot the minimum voltage detected among the various diode strings. A moredetailed implementation of a voltage comparator according to oneembodiment is discussed below in reference to FIG. 2.

Since the outputs of each of the voltage comparators are coupled to oneanother and are further fed back to one of the inputs (Vin2), only theamplifier 112 detecting the minimum voltage on its Vin1 input willoperate in a balanced fashion. The output transistor Mout of the“winning” amplifier (i.e. the amplifier with the minimum voltage on itsVin1) will sink excess current from the amplifiers 112 of the othervoltage comparators 105. As a result, Vout from the winning amplifierwill be the minimum voltage, Vmin, that is output from minimum voltagedetector 100.

FIG. 2 is a schematic diagram of one embodiment of a voltage comparatorcircuit. In the embodiment shown, voltage comparator 105 includes anamplifier 112 and a replica circuit 114. Amplifier 112 in the embodimentshown is an operational transconductance amplifier (OTA), although thedisclosure is not limited to this type of amplifier. Replica circuit 114in the embodiment shown may detect when amplifier 112 is operating in anunbalanced manner, which may occur when the first input voltage, Vin1,is not the minimum voltage detected from among the various diodestrings.

Amplifier 112 in the embodiment shown includes two input transistors,Min1 and Min2. The gate terminal of Min1 may be coupled to an LED stringhaving a plurality of LEDs coupled in series. More particularly, Vin1 isan input coupled to a cathode of a last LED in the string. A secondinput, Vin2 on the gate terminal of Min2, is coupled to an output node,Vout, of amplifier 112. Respective source terminals of Min1 and Min2 areeach coupled to receive a bias current from a bias current source,Ibias. The bias current source is coupled to receive a supply voltage,Vdd_Ana.

An output transistor, Mout, includes a drain terminal that is coupled toa drain terminal of the second input transistor, Min2. A gate terminalof Mout is coupled to a drain terminal of Min1. A capacitor C is coupledbetween the respective drain terminals of Min1 and Min2. A secondcurrent source, Ibias/2, is coupled between the drain terminal of Min1and a ground node. The output transistor, meanwhile, sinks an outputcurrent, Iout. When operation of amplifier 112 is balanced, the outputcurrent, Iout, is equal to the current through current source Ibias/2.In other words, during balanced operation, each leg of amplifier 112sinks one half of the bias current, Ibias, which occurs when the inputsare equal. This condition could also occur if only a single comparatorwere connected to amplifier 112.

The output voltage from the amplifier is taken from the drain terminalof Mout. This output voltage is also fed back to amplifier 112, and inparticular, to the Vin2 input on the gate terminal of Min2.

Replica circuit 114 in the embodiment shown includes a sense transistor,MSense. The gate terminal of MSense is coupled to the gate terminal ofMOut in amplifier 112, as well as the drain terminal of Min1. A drainterminal of MSense is coupled to a current source, Ibias/2−Ioff. Thecurrent Ioff corresponds to an amount of an offset current needed toensure the replica circuit is turned off when the amplifier is balanced.This current may be sized to cover a worst case process mismatch betweenthe source of Ibias/2 and Iout, which in turn is coupled to receive asupply voltage from Vdd_Ana. In the embodiment shown, MSense is areplica of Mout, being substantially the same size and havingsubstantially the same operating characteristics.

Transistor MClamp includes a gate terminal coupled to the drain terminalof MSense, with the source terminal of MClamp being coupled to the gateterminal of Mout. A drain terminal of MClamp is coupled to a drainterminal of M1, which is diode-coupled in the embodiment shown. Therespective gate terminals of M1 and M2 are coupled to one another, andthus form a current mirror. The drain terminal of M2 is coupled toanother current source, Ibias/4. An inverter, Inv1, includes an inputterminal coupled to the drain terminal of M2. When the output frominverter Inv1, ‘min’, is asserted, voltage comparator 105 is providingan indication it is the comparator that is detecting the minimumvoltage.

When Vin1 is not the minimum voltage detected among a number of voltagecomparators 105, amplifier 112 is unbalanced in operation. This is dueto the fact that the gate voltage Vin1 on transistor Min1 is differentthan the gate voltage on Vin2, which is coupled to Vout and thus to theoutput nodes of the other comparators 105 (and thus lower). Whenunbalanced due to Vin1 being greater than Vin2, a lower amount ofcurrent is sunk through the left branch of amplifier 112 relative to theright branch. As a result, the gate voltage on Mout, and thus on Msense,falls. As a result, Msense sinks less current, and thus the voltage onits drain terminal rises. Correspondingly, the voltage on the gateterminal of MClamp rises. When the gate voltage of Mout/MSense fallswhile the gate voltage of MClamp rises, the gate-source voltage of thelatter device may exceed its threshold voltage. When the gate-sourcevoltage of MClamp exceeds its threshold voltage, the device is turnedon. Since the source terminal is coupled to the respective gateterminals of Mout and Msense, the gate voltages of the latter twodevices are prevented from falling to zero. Accordingly, transistor Moutremains turned on, conducting a small amount of current, even thoughamplifier 112 is unbalanced (e.g., operating in a lower portion of itslinear region).

Since replica circuit 114 as described herein causes MOut to remain onand sinking a small amount of current even when amplifier 112 isunbalanced, the transistors used to implement MOut may be smaller thanembodiments in which no replica circuit is provided. In the absence ofreplica circuit 114, the gate-source voltage of Mout would fall to zero,and thus the device would turn off. This would result in a longer timeto turn on in the event that the operation changed from unbalanced tobalanced. Furthermore, in embodiments in which only the amplifier ispresent (e.g., no replica circuit), transistor Mout sinks the biascurrent from the amplifiers of the other voltage comparators. This inturn would require a larger device to implement Mout, and would furtherincrease the turn on time. However, since Mout in the embodiment of FIG.2 does not turn off when amplifier 112 is unbalanced (due to theoperation of the clamp transistor, MClamp), Mout of the winning celldoes not need to sink the entire bias current from all other cells.Accordingly, Mout in the embodiment of FIG. 2 can be implemented with asmaller device. Furthermore, since Mout does not completely turn off,transitioning from unbalanced to balanced operation may occur fasterrelative to similar circuits in which Mout is turned off when unbalancedoperation occurs.

When Vin1 is the minimum voltage detected among a number of voltagecomparators 105, amplifier 112 may be balanced in operation, asVin1=Vin2=Vout. The current through both branches of amplifier 112during balanced operation is substantially equal (e.g., the currentsthrough transistors Min1 and Min2 are equal). In addition to thesubstantially equal currents in both branches, the gate voltage on Mout,and thus Msense, is higher, with the source voltage of MClamp beinghigher as a result. Thus, with MSense sinking a greater amount ofcurrent, its drain voltage, and thus the gate voltage of MClamp, islower. Therefore, the gate-source voltage of MClamp is less than itsthreshold voltage, and this device remains off. While amplifier 112 andreplica circuit 114 illustrate the use of transistors M1, M2, Min1,Min2, one of ordinary skill in the art will recognize that one or moreof these transistors may be replaced by another type of switchingdevice.

FIG. 3 is a diagram illustrating one embodiment of a minimum voltagedetection circuit. More particularly, FIG. 3 illustrates the connectionof the LED strings 120, LED drivers 210, and voltage comparators 105.

As shown in FIG. 3, minimum voltage detector 100 includes a number ofvoltage comparators 105. Each of the voltage comparators 105 includes anoutput node, Vout, which is coupled to the corresponding output node ofevery other voltage comparators 105 implemented in minimum voltagedetector 100. In each instance of a voltage comparator 105, the outputnode, Vout, is coupled to one of the input nodes, Vin2. This arrangementcorresponds to that which is illustrated in FIG. 2.

Each of the voltage comparators 105 includes another input node, Vin1.Each of these input nodes is coupled to a corresponding LED string 120(although only one is shown here for the sake of simplicity). Moreparticularly, the Vin1 node is coupled to a cathode of a last diode inthe LED string 120. An LED driver circuit 210 is also coupled to theinput node Vin1. The LED driver circuit 210 may, through variousoperational modes, control the illumination of the LEDs in itscorrespondingly coupled LED string 120. For full brightness, the LEDdriver circuit 210 may operate with the illustrated transistorcontinuously turned on (this may be referred to as the continuous modeof operation). For reduced brightness, the LED driver circuit 210 mayoperate in a pulse width modulation (PWM) mode. Operation in the PWMmode includes turning the transistor on and off for various amounts oftime (thereby modulating the width of current pulses drawn through LEDstring 120), and may be used to reduce the apparent brightness of theLEDs in LED string 120.

It is noted that the LED driver circuit 210 shown in FIG. 3 is asimplified example. Any suitable embodiment of an LED driver circuit maybe used, including embodiments that are more complex than the exampleshown here.

FIG. 4 is a schematic diagram illustrating one embodiment of a faultdetection circuit. In the embodiment shown, fault detection circuit 405includes comparators 410 and 411, a control circuit 430, a low passfilter 415 (including resistor Rf and capacitor Cf), and a voltagedivider including resistors Rd1 and Rd2. The low pass filter 415provides minimum voltage buffering and smooths the transient response ofthe minimum voltage detector 100 when the minimum input changes (e.g.,the minimum voltage changes from being detected by one comparator 105 toanother). Comparator 411 includes a first input coupled to the junctionof Rd1 and Rd2, and a second input coupled to receive a thresholdvoltage ovl_th. Comparator 410 is coupled to receive, on one input, theoutput of the low pass filter 415, which in turn is coupled to the Vminoutput of minimum voltage detector 100. The other input of comparator410 is coupled to receive another threshold voltage, uv_th.

Detection of a fault and the triggering of a check for opens and shortsin an LED string 120 is triggered when the output of comparator 411OVL=1 and the output of comparator 410, UV=1. The OVL=1 condition occurswhen the voltage at the junction of Rd1 and Rd2 exceeds that of thethreshold voltage ovl_th. The UV=1 condition occurs when the voltageoutput from the low pass filter 415 (which receives Vmin as its input)is less than a threshold voltage.

When both OVL=1 and UV=1, the presence of a fault is indicated. However,the nature of the fault, open circuit or short circuit, is unknown uponinitial detection. The OVL and UV signals may be provided to controlcircuit 430, as well as to an external destination.

Determination of whether the fault is a short or an open may beperformed as follows. When either a short circuit or open circuit occursin a diode string, its voltage will be zero on the input to itscorresponding voltage comparator 105. The voltage comparator 105detecting the zero minimum voltage may thus provide an indication tocontrol circuit 430, e.g., through a corresponding one of signalsMin[17:0] (which corresponds to the ‘min’ signal output from an instanceof replica circuit 114, as shown in FIG. 2). Upon determining that ashort or open is present, control circuit 430 may generate controlsignals to isolate the fault and determine whether the fault is an openor short.

Control circuit 430 in the embodiment shown may assert an enable signaland a sample signal corresponding to the LED string 120 indicated asfaulty. Since the voltage is either zero or very low on the cathode ofthe last diode of the faulty LED string 120 (and thus passing throughthe passgate transistor Mpu), the input to the corresponding voltagecomparator 105 needs to be pulled high. This is accomplished via thecorresponding current source I_pu. The enable signal may be provided toa current source I_pu (e.g., en_pu0 to the first instance of I_pu asshown in FIG. 4). Meanwhile, the corresponding sample transistor, Ms maybe activated. Thus, when on, the current source I_pu attempts to pullhigh the Vin input of the corresponding voltage comparator 105.

With the current source I_pu attempting to pull up the Vin input of thecorresponding comparator, an output of UV=1 from comparator 410indicates the presences of a short circuit. This is due to the output ofthe corresponding voltage comparator Vout, and thus the output from lowpass filter 415, being zero or a very low voltage that is less than thethreshold uv_th. The short circuit condition in this case prevents theinput voltage Vin to the corresponding voltage comparator 105 beingpulled sufficiently high to raise the output voltage to a point whereinthe low pass filter output can exceed the threshold voltage.

If the output of comparator 410 is UV=0 when current source I_puattempting to pull up the Vin input of the corresponding comparator, thefault detection circuit 405 is indicating the presence of an opencircuit. When UV=0, the voltage on Vmin is sufficiently high that theoutput of low pass filter 415 exceeds the threshold voltage uv_th. Thisindicates that the current source I_pu is able to successfully pull upthe Vin1 input of the corresponding voltage comparator, which ispossible when an open circuit is present but not possible for a shortcircuit.

Responsive to detecting a fault and determining whether the fault is ashort circuit or an open circuit, various actions may be taken. If thefault is an open circuit, LED driver circuit 210 for the correspondingLED string 120 may be shut down, while the corresponding voltagecomparator may be isolated via by turning off the corresponding sampletransistor Ms.

FIG. 5 diagram illustrating one embodiment of a backlight system havinga number of LED strings and a DC-DC converter. In the embodiment shown,backlight system 500 includes a DC-DC converter 505, a number of LEDstrings 120, a minimum voltage detector circuit 100, a fault detectioncircuit 405 (which may operate in accordance with the discussion above),a digital dimming circuit 520, and a number of LED driver circuits 210(shown here as a single block for simplicity).

Digital dimming circuit 520 in the embodiment shown is arranged tocontrol the LED driver circuits 210. Among the control functionsperformed by digital dimming circuit 520 is controlling the mode ofoperation of the various LED driver circuits 210 (e.g., PWM orcontinuous mode). Other inputs to digital trim circuit 520 may includean indication of a desired brightness (e.g., the “Brightness” input).The digital dimming circuit 520 may generate one or more control signalsprovided to LED driver circuits to control the mode of operation and thebrightness of the backlight.

DC-DC converter 505 in the embodiment shown is configured to provide asupply voltage VddB to the anode of a first LED in each of the LEDstrings 120. In this particular embodiment, DC-DC converter 505 is aboost converter configured to output the supply voltage VddB at a valuegreater than the source input voltage Vsrc. The source voltage isprovided to a first terminal of an inductor L, while a second terminalof the inductor L is coupled to a drain terminal of transistor Mb.Transistor Mb in the embodiment shown is switched off and on by PWM/PFMcontrol circuit 515. Accordingly, transistor Mb may be switched inaccordance with a PWM mode, or a PFM (pulse frequency modulation mode).The voltage on the junction Mb and inductor L is conveyed to the anodeof diode Db, which outputs the supply voltage VddB.

Error amplifier 510 in the embodiment shown is coupled to receive theminimum voltage, Vmin, from the minimum voltage detector 100. The erroramplifier 510 may also receive a reference voltage, Vref. Based on adifference between the minimum voltage, Vmin, and the reference voltage,Vref, error amplifier 510 may generate an error signal that is input inPWM/PFM control circuit 515. Based on the error signal, PWM/PFM 515 mayadjust the switching of transistor Mb to cause DC-DC converter 505 toprovide a desired value of output voltage VddB.

Various embodiments of the minimum voltage detector circuit discussedabove may provide certain advantages over previous minimum voltagedetectors. Using various embodiments of the circuitry disclosed herein,a minimum voltage detector may be implemented for a larger number of LEDstrings than previous embodiments. For example, while previous minimumvoltage detectors may be limited to, e.g., 8 LED strings, the minimumvoltage detector disclosed herein may be implemented for a significantlylarger number (e.g., 18) LED strings. The die area consumed by thesecircuits may also be reduced on a per-comparator basis, since the outputtransistor (Mout) of the various embodiments discussed above may beimplemented using a smaller device relative to those in which no replicacircuit is provided. Meanwhile, the operation of the replica circuit,which keeps the output transistor of the amplifier turned on even whenunbalanced allows for faster changes between balanced and unbalancedoperation due to the minimum voltage changing from one LED string to thenext.

FIG. 6 is a flow diagram illustrating one embodiment of a method foroperating a voltage comparator circuit. The voltage comparator circuitdiscussed herein may be any embodiment of that which is discussed above,and may be implemented in a minimum voltage detector circuit.Embodiments of a voltage comparator circuit not explicitly discussedherein but capable of carrying out the functions discussed below mayfall within the scope of this disclosure.

Method 600 begins with receiving, on a first input of a particular oneof a plurality of voltage comparators, an input voltage from acorresponding on of a plurality of light-emitting diode (LED) stringseach comprising a plurality of series-coupled LEDs, wherein each of theplurality of voltage comparators includes an amplifier and a replicacircuit coupled to the amplifier (block 605). The method furtherincludes generating an output voltage on an output of the particular oneof the plurality of voltage comparators, wherein the output and a secondinput of the particular one of the plurality of voltage comparators iscoupled to respective outputs of each of the other ones of the pluralityof voltage comparators (block 610). When implemented in a minimumvoltage detection circuit, the voltage on an input of the circuit may bethe minimum voltage of call comparators (block 615, yes), or may beabove the minimum voltage (block 615, no). Responsive to detecting thatthe input voltage received on the first input of the plurality ofvoltage comparators is a minimum voltage received from the plurality ofLED strings, the method includes holding a clamp transistor in thereplica circuit in an inactive state (block 620). Additionally, theoutput of the amplifier is forced to a value equal to the input.Responsive to detecting that the input voltage received on the firstinput of the plurality of voltage comparators is not the minimum voltagereceived from the plurality of LED strings, the method includesactivating the clamp transistor (block 625).

In various embodiments, detecting that the input voltage received on thefirst input of the plurality of voltage comparators is a minimum voltagereceived from the plurality of LED strings comprises determining thatthe amplifier is in a balanced state. Detecting that the input voltagereceived on the first input of the plurality of voltage comparators isnot the minimum voltage received from the plurality of LED stringscomprises determining that the amplifier is in an unbalanced state. Theparticular one of the plurality of voltage comparators may also providean indication responsive to detecting that the input voltage received onthe first input of the plurality of voltage comparators is the minimumvoltage received from the plurality of LED strings.

In various embodiments, the method includes detecting presence of acircuit fault in one of the LED strings based on an output voltage froma corresponding one of the plurality of voltage detectors. The methodmay also include determining whether the circuit fault is a shortcircuit or an open circuit. The method may further include a DC-DCconverter providing a supply voltage to an anode of a first LED in eachof the plurality of LED strings and the DC-DC converter adjusting thesupply voltage based on the minimum voltage received from the pluralityof LED strings.

Turning next to FIG. 7, a block diagram of one embodiment of a system150 is shown. In the illustrated embodiment, the system 150 includes atleast one instance of an integrated circuit 10 coupled to externalmemory 158. The integrated circuit 10 may include a memory controllerthat is coupled to the external memory 158. The integrated circuit 10 iscoupled to one or more peripherals 154 and the external memory 158. Apower supply 156 is also provided which supplies the supply voltages tothe integrated circuit 10 as well as one or more supply voltages to thememory 158 and/or the peripherals 154. In some embodiments, more thanone instance of the integrated circuit 10 may be included (and more thanone external memory 158 may be included as well).

The peripherals 154 may include any desired circuitry, depending on thetype of system 150. For example, in one embodiment, the system 150 maybe a mobile device (e.g. personal digital assistant (PDA), smart phone,etc.) and the peripherals 154 may include devices for various types ofwireless communication, such as WiFi, Bluetooth, cellular, globalpositioning system, etc. The peripherals 154 may also include additionalstorage, including RAM storage, solid-state storage, or disk storage.The peripherals 154 may include user interface devices such as a displayscreen, including touch display screens or multitouch display screens,keyboard or other input devices, microphones, speakers, etc. In otherembodiments, the system 150 may be any type of computing system (e.g.desktop personal computer, laptop, workstation, tablet, etc.).

The external memory 158 may include any type of memory. For example, theexternal memory 158 may be SRAM, dynamic RAM (DRAM) such as synchronousDRAM (SDRAM), double data rate (DDR, DDR2, DDR3, LPDDR1, LPDDR2, etc.)SDRAM, RAMBUS DRAM, etc. The external memory 158 may include one or morememory modules to which the memory devices are mounted, such as singleinline memory modules (SIMMs), dual inline memory modules (DIMMs), etc.

System 150 in the embodiment shown also includes at least one instanceof a display 157, the display including a backlight. The backlight ofdisplay 157 may be implemented using a number of LED strings, each ofwhich is coupled to a minimum voltage detection circuit as discussedabove. The minimum voltage detection circuit may utilize variousembodiments of the voltage comparator circuit discussed above. Faultdetection as discussed above may also be implemented.

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

What is claimed is:
 1. A circuit comprising: a minimum voltage detectorcircuit comprising a plurality of voltage comparators, wherein theminimum voltage detector circuit is configured to detect a minimumvoltage from among a plurality of LED strings each comprising aplurality of series-coupled light-emitting diodes (LEDs), wherein theminimum voltage detector circuit includes a plurality of voltagecomparators, each of the voltage comparators including: an amplifierhaving a first input coupled to a cathode of a last LED of one of theplurality of LED strings, an output, and a second input coupled to theoutput; and a replica circuit coupled to the amplifier, wherein thereplica circuit is configured to maintain an output transistor of theamplifier in an active state when the amplifier is in an unbalancedstate.
 2. The circuit of claim 1, wherein the replica circuit includes asense transistor having a respective gate terminal coupled to a gateterminal of the output transistor of the amplifier, wherein the sensetransistor is configured to sink an amount of current that correspondsto an amount of current through the output transistor.
 3. The circuit ofclaim 2, further comprising a clamp transistor having a gate terminalcoupled to a drain terminal of the sense transistor, and a sourceterminal coupled to respective gate terminals of the sense transistorand the output transistor, wherein the sense transistor is configured tocause activation of the clamp transistor responsive to the currentthrough the output transistor being less than a threshold, and furtherconfigured to cause the clamp transistor to be inactive when the currentthrough the output transistor is equal to or greater than the threshold.4. The circuit of claim 1, wherein respective outputs of amplifiers ofeach of the plurality of voltage comparators are coupled to an output ofthe minimum voltage detector circuit.
 5. The circuit of claim 4, whereinthe minimum voltage detector circuit is configured to output a voltagecorresponding to a lowest voltage detected by a particular one of theplurality of voltage comparators.
 6. The circuit of claim 1, wherein theamplifier of each voltage comparator is an operational transconductanceamplifier (OTA), and wherein the circuit further includes a plurality ofLED driver circuits each coupled to a first input of an OTA of acorresponding one of the plurality of voltage comparators.
 7. Thecircuit of claim 1, wherein the replica circuit in each of the pluralityof voltage comparators is configured to provide an indication if thatparticular one of the plurality of voltage comparators is detecting theminimum voltage from among the plurality of LED strings.
 8. The circuitof claim 1, further comprising a fault detection circuit coupled to eachof the plurality of voltage comparators, wherein the fault detectioncircuit is configured to detect whether a particular one of theplurality of LED strings has a short circuit or an open circuit based ona comparison between a threshold voltage and an output voltage receivedfrom a corresponding one of the plurality of voltage comparators.
 9. Thecircuit of claim 1, further comprising a DC-DC converter coupled toprovide a supply voltage to an anode of a first LED in each of theplurality of LED strings, wherein the DC-DC converter is configured toadjust the supply voltage based on the minimum voltage detected fromamong the plurality of LED strings.
 10. The circuit of claim 1, furthercomprising a plurality of LED driver circuits each coupled to acorresponding one of the plurality of LED strings and further coupled toa first input of an amplifier implemented in a corresponding one of theplurality of voltage comparators.
 11. A method comprising: receiving, ona first input of a particular one of a plurality of voltage comparators,an input voltage from a corresponding on of a plurality oflight-emitting diode (LED) strings each comprising a plurality ofseries-coupled LEDs, wherein each of the plurality of voltagecomparators includes an amplifier and a replica circuit coupled to theamplifier; generating an output voltage on an output of the particularone of the plurality of voltage comparators, wherein the output and asecond input of the particular one of the plurality of voltagecomparators is coupled to respective outputs of each of the other onesof the plurality of voltage comparators; responsive to detecting thatthe input voltage received on the first input of the plurality ofvoltage comparators is a minimum voltage received from the plurality ofLED strings, holding a clamp transistor in the replica circuit in aninactive state; and responsive to detecting that the input voltagereceived on the first input of the plurality of voltage comparators isnot the minimum voltage received from the plurality of LED strings,activating the clamp transistor.
 12. The method of claim 11, whereindetecting that the input voltage received on the first input of theplurality of voltage comparators is a minimum voltage received from theplurality of LED strings comprises determining that the amplifier is ina balanced state, and wherein detecting that the input voltage receivedon the first input of the plurality of voltage comparators is not theminimum voltage received from the plurality of LED strings comprisesdetermining that the amplifier is in an unbalanced state.
 13. The methodof claim 11, further comprising the particular one of the plurality ofvoltage comparators providing an indication responsive to detecting thatthe input voltage received on the first input of the plurality ofvoltage comparators is the minimum voltage received from the pluralityof LED strings.
 14. The method of claim 11, further comprising:detecting presence of a circuit fault in one of the LED strings based onan output voltage from a corresponding one of the plurality of voltagecomparators; and determining whether the circuit fault is a shortcircuit or an open circuit.
 15. The method of claim 11, furthercomprising: a DC-DC converter providing a supply voltage to an anode ofa first LED in each of the plurality of LED strings; and the DC-DCconverter adjusting the supply voltage based on the minimum voltagereceived from the plurality of LED strings.
 16. An apparatus comprising:a DC-DC converter configured to generate a supply voltage; a pluralityof light-emitting diode (LED) strings each including a plurality of LEDscoupled in series, wherein an anode of a first LED in each of theplurality of LED strings is coupled to receive the supply voltage; and aminimum voltage detector circuit having a plurality of comparators andconfigured to detect a minimum voltage that is output from among theplurality of LED strings, wherein each of the plurality of comparatorsincludes: an amplifier having a first input coupled to a cathode of alast LED in a corresponding one of the plurality of LED strings, and anoutput transistor, wherein an output of the amplifier is coupled to anoutput of amplifier implemented in each other one of the plurality ofcomparators; and a replica circuit having a sense transistor coupled tothe output transistor, and a clamp transistor, wherein the sensetransistor is configured to cause activation of the clamp transistorresponsive to determining the amplifier is in an unbalanced state. 17.The apparatus of claim 16, wherein the amplifier of each of theplurality of comparators is configured to operate in a balanced state ifa voltage on its first input is the minimum voltage that is output fromamong the plurality of LED strings, and wherein the amplifier of each ofthe plurality of comparators is further configured to operate in theunbalanced state if the voltage on its first input is not the minimumvoltage that is output from among the plurality of LED strings.
 18. Theapparatus of claim 16, wherein an output of the amplifier of each of theplurality of comparators is coupled to a second input of that amplifier,and further coupled to an output of an amplifier of each other one ofthe plurality of comparators.
 19. The apparatus of claim 16, furthercomprising a fault detection circuit coupled to each of the plurality ofcomparators, wherein the fault detection circuit is configured to detecta presence of a fault in any particular one of the plurality of LEDstrings.
 20. The apparatus of claim 16, wherein the minimum voltagedetector circuit is coupled to provide a voltage corresponding to theminimum voltage that is output from among the plurality of LED stringsto the DC-DC converter, wherein the DC-DC converter is control thesupply voltage based on the voltage provided by the minimum voltagedetector circuit.